Melatonin is a hormone produced by the pineal gland at night. The timing of its release is controlled by the circadian timing system. Its timing can be suppressed or reset by exposure to light through a multisynaptic pathway from the eye through the suprachiasmatic nucleus, the master circadian clock. Exogenous melatonin supplementation has been reported to act in two ways in humans: as a soporific (sleep-promoting) and a chronobiotic (to reset circadian phase), thus affecting both of the neurobiologic systems that interact to regulate sleep and wakefulness. Melatonin supplements have been reported to be an effective treatment for circadian rhythm sleep disorders, including shift work dyssomnia, jet-lag, delayed sleep phase syndrome, and sleep disruption suffered by many blind individuals. However, in a recent meta-analysis, the vast majority of these reports were excluded because they were not randomized controlled trials. The conclusion was that melatonin is not efficacious for sleep complaints associated with jetlag and shift work. In view of the widespread, uncontrolled, over-the-counter use of melatonin for treating sleep complaints in the general population, a randomized controlled trial of melatonin is essential to evaluate efficacy in treatment of transient insomnia. The current proposal will test the chronobiotic and sleep promoting effects of melatonin in a phase advance model of insomnia. Subjects will be administered either melatonin or placebo before a scheduled sleep episode which is advanced by several hours, simulating eastward travel. Under these conditions, low sleep pressure and the timing of the circadian clock make sleep particularly difficult. We will measure sleep and multiple physiologic rhythms controlled by the circadian after melatonin or placebo administration, and use precise analytic methods to determine the impact of melatonin on sleep and the human circadian timing system. This study will provide important information regarding the mechanism of action of melatonin that will be critical for the use of melatonin as a treatment for circadian rhythm sleep disorders.